[DWARF] Show which augmentation character was unrecognized.

[MaskRay]

LGTM. The issues should be rare but adding more information has little overhead for this diagnostic.

llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp

if (Id == getCIEId(IsDWARF64, IsEH)) {
         uint64_t EndAugmentationOffset;
 
         // Walk the augmentation string to get all the augmentation data.
         for (unsigned i = 0, e = AugmentationString.size(); i != e; ++i) {
           switch (AugmentationString[i]) {
           default:
             return createStringError(
                 errc::invalid_argument,
-                "unknown augmentation character in entry at 0x%" PRIx64,
-                StartOffset);
+                "unknown augmentation character %c in entry at 0x%" PRIx64,
+                AugmentationString[i], StartOffset);
           case 'L':
             LSDAPointerEncoding = Data.getU8(&Offset);
             break;
           case 'P': {
             if (Personality)
               return createStringError(
                   errc::invalid_argument,
                   "duplicate personality in entry at 0x%" PRIx64, StartOffset);